This invention relates to an apparatus for depositing thin films of material and, in particular, to an apparatus for continuous deposition of thin films by vacuum evaporation. Such apparatus is particularly useful for the continuous manufacture of thin film solar cells.
Thin film solar cells possess many potential advantages for use in converting solar energy to electrical power. Thin film cells employing a film of cadmium sulfide converter and a copper sulfide absorber are flexible, light of weight and can be made with commercially acceptable levels of conversion efficiency. Moreover, as contrasted with silicon PN junction solar cells which must be fabricated by inefficient batch processes, there is considerable promise that thin film solar cells can be commercially manufactured using continuous processing techniques.
A key step in the continuous manufacture of thin film cadmium sulfide solar cells is the vacuum evaporation of the cadmium sulfide collector onto a temperature-controlled moving substrate. In large-scale production, cost considerations require that the cadmium sulfide be substantially uniformly deposited on the substrate with relatively little wastage of the evaporated material. In addition, economic considerations also require that large substrate areas be continuously coated without breaking vacuum. Moreover, the rate of deposition must be controlled in the direction of substrate movement and be uniform perpendicular to this direction in order to obtain the required mechanical and electrical properties in the film. Typical prior art vacuum evaporation arrangements are not well-suited to this task. Such arrangements typically feed a wire or ribbon of the material to be coated to an evaporator. The rate of evaporation is controlled by the evaporator temperature and the rate of wire feed. To obtain uniform deposition coating, the evaporated material is applied through a large area orifice through a relatively long source-to-substrate distance.
Such arrangements are not suitable for the large-scale manufacture of cadmium sulfide solar cells. Cadmium sulfide is a sublimable powder and cannot be readily formed into wire or ribbon. Since cadmium sulfide cannot be fed to the evaporator in the form of a liquid or wire, prior solutions involved loading cadmium sulfide powder, pellets or sintered cakes into crucibles. The need to periodically reload the crucibles either precludes continuous operation or necessitates mechanically complex loading and transfer machinery. Thus, the conventional technique for continuous feed of source material is not easily applicable. Control of evaporation rate from a multitude of crucibles by maintaining the temperature of each crucible is difficult and costly. Thus the conventional technique for rate control is inapplicable. Moreover, systems using large source-to-substrate distances are very wasteful of the evaporated material because much of the material misses the substrate and coats the walls of the chamber.